A two-phase argon detector is being developed in our laboratory for dark matter search and low-energy neutrino experiments. A 109Cd γ-ray source was used to calibrate the energy scale of the detector. In this paper, a detailed emission spectrum of the 109Cd source was measured using a YAP:Ce based scintillation detector and high-purity germanium detector. It was shown that the 109Cd source equipped with a tungsten substrate and a copper filter can provide a complete set of γ-ray lines in the range from 8 to 90 keV for energy calibration of two-phase detectors. These measurements allowed us to successfully quantify the shape of the amplitude spectrum observed when the two-phase detector was irradiated by the 109Cd source.